TWI435445B - Method for doping non-damaging impurities of complementary MOS image sensors - Google Patents

Description

Method for doping non-damaging impurities of complementary MOS image sensors

The present invention is generally directed to image sensors, and in particular, but not exclusively, to back-illuminated ("BSI") image sensors having improved back surface doping.

Image sensors have become ubiquitous. Image sensors are widely used in digital still cameras, cellular phones, security cameras, medical, automotive and other applications. The technology used to make image sensors (and, in particular, CMOS image sensors ("CIS")) has continued to advance. For example, the need for higher resolution and lower power consumption has contributed to further miniaturization and integration of image sensors. Therefore, the number of pixels in the pixel array of the image sensor has increased, and the size of each pixel unit has decreased.

Typically, each pixel of the image sensor includes a photosensitive element such as a photodiode, and one or more transistors for reading signals from the photosensitive element. As the pixel unit size decreases, the transistor size may also decrease. Transfer transistors are commonly used in pixels with a four-transistor design. The transfer transistor separates the photosensitive element from the rest of the pixel. A transfer transistor is formed between the photosensitive element and a floating node, and it is desirable to scale down the transfer transistor to have a short gate length, thereby achieving a larger integrated and enhanced pixel fill factor.

In most image sensors, constituent elements of each pixel are formed on or near a surface considered to be the front surface of the substrate, and light captured by the pixels is incident on the front surface. Instead of capturing light incident on the front side of the substrate, or in addition to capturing light incident on the front side of the substrate, some image sensors, referred to as back-illuminated (BSI) image sensors, can be captured after being incident on the substrate. Light on the surface. In BSI image sensors, back illumination causes most of the photon absorption to occur near the surface of the back side. In order to separate the pair of electron holes generated by photon absorption and drive the electrons toward the photosensitive region, an electric field near the rear surface of the crucible is helpful. This electric field can be generated by doping the back surface of the crucible. The quality of the surface after doping plays an important role in image sensor performance. Crystal defects and inactive dopants in the surface region after doping can degrade quantum efficiency by trapping electrons and not allowing electrons to reach the photosensitive region, which can result in "hot pixel" defects.

One of the main sources of crystal defects in CMOS image sensors is the result of conventional ion implantation processes involving dopant implantation followed by thermal annealing to implant the implanted dopants. Agent activation. Thermal laser annealing is a method used to reduce the occurrence of crystal defects after ion implantation, but laser annealing causes local heating, which can significantly increase the substrate temperature of BSI CIS due to image sensing in these types. In the device, an epitaxial (epi) layer (pixels are mainly formed therefrom) is thin (for example, <4 μm thick). An increase in substrate temperature can result in undesirable dopant diffusion and/or metal degradation/melting. The possibility of overheating in undesired regions can be reduced by using a thicker final epitaxial layer that can be created by removing less epitaxial layers during the substrate thinning process. However, increasing the thickness of the epitaxial layer results in an increase in electrical crosstalk between adjacent pixels in the image sensor.

In addition to causing an increase in substrate temperature, laser annealing may not activate all of the backside dopants, which can result in inactive dopant defects. Such problems associated with current fabrication procedures using ion implantation and laser thermal annealing can cause undesirable problems in the resulting image sensor, such as high dark currents and high white pixel counts.

The non-limiting and non-exhaustive embodiments of the present invention are described with reference to the accompanying drawings, wherein, unless otherwise indicated,

Embodiments of a method for producing a non-damaging impurity doping of a back-illuminated ("BSI") image sensor are described herein. In the following description, numerous specific details are set forth in the description of the embodiments of the invention The present invention is practiced in terms of components, materials, and the like. In other instances, well-known structures, materials, or operations, although not shown or described in detail, are still within the scope of the invention.

The reference to "an embodiment" in this specification means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, appearances of the phrase "in an embodiment" Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. The directional terminology such as "top", "bottom", "under" is used in reference to the orientation of the described drawings, and should not be construed as limiting the scope of the embodiments.

1 is a cross-sectional view of an embodiment of an image pixel 100 of a backside illuminated (BSI) image sensor having a backside dopant. The illustrated embodiment of image pixel 100 is one of the pixels within the pixel array of the image sensor. The illustrated embodiment of image pixel 100 includes a p-type epitaxial layer 110, a p-type pinned layer 117, an n-type photosensitive region 115, a transfer transistor 120, and shallow trench isolation formed on the front side of the epitaxial layer 110. ("STI") and n+ source/drain diffusion region 130. N+ source/drain diffusion region 130 is formed within p-well 140. Of course, in other embodiments, the conductivity type of the elements of pixel 100 can be reversed. For example, in an alternate embodiment, regions 115, 130 may be P-doped while regions 110, 140, and 116 may be N-doped.

The interlayer dielectric 154 separates the front surface of the epitaxial layer 110 from the metal stack 150. Although FIG. 1 illustrates a two-layer metal stack, the metal stack 150 can include more or fewer metal layers for directing electrical signals through the front side of the pixel array. The back p+ doped layer 116 is formed on the back side of the epitaxial layer 110 having a thickness L. An optional anti-reflective ("AR") layer 160 can be formed on the back side of the back p+ doped layer 116. Color filter 165 may be formed on the back side of AR layer 160 and may have a Bayer pattern, or an infrared filter, or a combination of the two. In other embodiments, color filter 165 may be completely absent. The microlens 170 is formed on the back surface of the image pixel 100, and guides light from the back surface to the n-type photosensitive region 115.

2 is a flow chart illustrating an embodiment of a process 200 for fabricating image pixels 300 (see FIGS. 3A-3D) of a BSI image sensor. The order of some or all of the program blocks appearing in program 200 should not be considered limiting. Rather, those skilled in the art having the benefit of this disclosure will appreciate that some of the program blocks can be performed in various sequences not illustrated.

In program block 205, the fabrication of image pixels 300 is performed prior to the fabrication of the Back End Process ("BEOL") component, which includes diffusion implants, germanium, pixel transistor circuits, and the like. The metal stack 350 is shown in Figure 3A. FIG. 3A also shows epitaxial layer 310 formed on p+ substrate 305. In program block 207, a handle wafer (not shown in Figures 3A-3D) is bonded to the front image sensor wafer. In program block 210, the back side of image pixel 300 is thinned to remove p+ substrate 305 and expose the back side of p-type epitaxial layer 310.

In block 215, the pure p-type dopant region 390 is formed in or on the backside of the p-type epitaxial layer 310 using a plasma immersion ion implantation ("PIII") process (see Figure 3B). As used herein, the term "pure" as used with respect to a dopant region does not mean that the dopant region must be made of 100% dopant selected without impurities. The truth is, "pure" means that the dopant region is mainly composed of selected dopants but may contain any amount of one or more impurities as long as the impurities do not appear in the following amounts: this amount will interfere with the use of the PIII process. The formation of the dopant region 390 interferes with the later formation of the back doped layer 316 or interferes with the operation of the resulting pixel. In PIII, the surface of the p-type epitaxial layer 310 is exposed to the plasma, and a high negative voltage is applied to form an electric field between the surface of the p-type epitaxial layer 310 and the plasma. This electric field accelerates the p-type dopant ions from the plasma toward the surface of the p-type epitaxial layer, thereby implanting the plasma. In an embodiment, the dopant ions can be boron, but in other embodiments, other types of dopants can be used. Using this process, a pure p-type dopant region 390 is formed on the back side of the p-type epitaxial layer 310. In general, dopant region 390 has a polarity opposite to that of photosensitive region 315: in the illustrated embodiment, pure dopant region 390 is p-doped and photosensitive region 315 is n-doped, but in the photosensitive region In the 315 p-doped embodiment, the pure dopant region 390 can be n-doped.

In program block 220, a PIII process similar to the PIII process used in program block 215 can be used to deposit cap film 395 on the back side of pure p-type dopant region 390, as shown in FIG. 3B. Show. In one embodiment, the cover film 395 can be a polysilicon, but in other embodiments it can be another form or completely another material. In program block 225, a laser pulse is applied to melt the back surface of cap film 395 and p-type epitaxial layer 310. After recrystallization of the p-type epitaxial layer, the pure dopant region 390 becomes incorporated into the epitaxial layer 310, resulting in very high dopant activation in the back p-doped layer 316, as shown in Figure 3C. . The laser pulse melts the pure p-type dopant region 390 and the cap film 395 in a few nanoseconds, and the back surface of the p-type epitaxial layer 310 recrystallizes in about 10 nanoseconds. When compared to conventional methods - ion implantation and thermal laser annealing combinations that may take several minutes to activate implanted ions - shorter laser annealing periods may reduce the depth of dopant diffusion and / or due to extended substrate The metal with increased temperature deteriorates/melts.

The depth of the back p+ doped layer 316 can be controlled by controlling the thickness of the pure p-type dopant region 390; in various embodiments, the back p+ doped layer 316 can have between about 0.1 microns and about 0.3 microns. thickness. The dopant concentration of the back p+ doped layer 316 can be controlled by the duration of the laser pulse melting and power. In an embodiment, the laser pulses may have a duration of less than 10 nanoseconds (ns), but may have different durations in other embodiments. Similarly, in various embodiments, doped layer 316 can have a dopant concentration between 10 ¹⁸ ions/cm 3 and 10 ²⁰ ions/cm 3 , although of course other dopant concentrations in other embodiments possible. After melting and recrystallization, a p-type junction is formed on the back side of the epitaxial layer 310. It is not necessary to remove any film or layer after the laser pulse has melted. In addition, the use of this process can reduce the occurrence of crystal defects and inactive dopants.

In program block 230, the fabrication of image pixels 300 is accomplished by the addition of anti-reflective (AR) layer 360, color filter 365, and microlens 370, as shown in Figure 3D. Note that Figures 1 and 3A through 3D show only a cross section of a single pixel within a pixel array. Thus, fabrication of the complete image sensor will include the fabrication of an array of color filters 365 and an array of microlenses 370, but the AR layer 360 can be a blanket layer that is shared by multiple repeating devices. It should be noted that the above description assumes that the image sensor uses the implementation of red, green, and blue photosensitive elements. Those skilled in the art having the benefit of this disclosure will appreciate that the description is also applicable to other basic or complementary color filters.

4 is a block diagram illustrating one embodiment of a BSI imaging system 400. The illustrated embodiment of imaging system 400 includes a pixel array 405, readout circuitry 410, functional logic 415, and control circuitry 420.

Pixel array 405 is a two-dimensional ("2D") array of back-illuminated imaging sensors or pixels (eg, pixels P1, P2...Pn). In one embodiment, each pixel is a complementary metal oxide semiconductor ("CMOS") imaging pixel and at least one pixel in the array can be one of the BSI pixel embodiments shown in FIGS. 1 and 3D. As illustrated, each pixel in the array is configured in columns (eg, columns R1 through Ry) and rows (eg, rows C1 through Cx) to obtain image data for a person, place, or object, which image data can then be used to present the person 2D image of location, location or object.

After each pixel has acquired its image data or image charge, the image data is read by readout circuit 410 and passed to function logic 415. Readout circuitry 410 can include an amplification circuit, analog to digital ("ADC") conversion circuitry, or other circuitry. Function logic 415 can simply store image data or even manipulate image data by applying post-image effects (eg, crop, rotate, red-eye, adjust brightness, adjust contrast, or other operations). In one embodiment, readout circuitry 410 can read a line of image material at a time along the readout line (described), or can read image data (not illustrated) using a variety of other techniques, such as serial readout or simultaneous The pixels are read in parallel.

Control circuit 420 is coupled to pixel array 405 to control the operational characteristics of pixel array 405. For example, control circuit 420 can generate a shutter signal for controlling image acquisition. In one embodiment, the shutter signal is a global shutter signal for enabling all of the pixels within pixel array 405 to simultaneously capture their respective image data during the signal acquisition window. In an alternate embodiment, the shutter signal is a scroll shutter signal that is sequentially read by the pixels of each column, row or group during successive acquisition windows.

5 is a circuit diagram illustrating an embodiment of two four-electrode ("4T") pixel pixel circuits 500 within a BSI imaging array, in accordance with an embodiment of the present invention. Pixel circuit 500 is a possible pixel circuit architecture for implementing each pixel in pixel array 405 of FIG. 4, but it should be understood that embodiments of the present invention are not limited to a 4T pixel architecture; Those of ordinary skill in the art will appreciate that the teachings are also applicable to 3T designs, 5T designs, and various other pixel architectures. In FIG. 5, the BSI pixels Pa and Pb are arranged in two columns and one row. The illustrated embodiment of each pixel circuit 500 includes a photodiode PD, a transfer transistor T1, a reset transistor T2, a source follower ("SF") transistor T3, and a select transistor T4. During operation, the transfer transistor T1 receives the transfer signal TX, and the transfer signal TX transfers the charge accumulated in the photodiode PD to the floating diffusion node FD. In an embodiment, the floating diffusion node FD may be coupled to a storage capacitor for temporarily storing image charges ^. The reset transistor T2 is coupled between the power rail VDD and the floating diffusion node FD to be reset under the control of the reset signal RST (eg, discharging or charging the FD to a preset voltage). The floating diffusion node FD is coupled to control the gate of the SF transistor T3. The SF transistor T3 is coupled between the power rail VDD and the selection transistor T4. The SF transistor T3 acts as a source follower that provides high impedance from the pixel output. Finally, the select transistor T4 selectively couples the output of the pixel circuit 500 to the readout row line under the control of the select signal SEL. In one embodiment, the TX signal, the RST signal, and the SEL signal are generated by control circuit 420.

The above description of the illustrated embodiments of the invention, including the description of the invention, is not intended to be It will be appreciated by those skilled in the art that the present invention may be described in the context of the present invention.

These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed as limiting the invention to the specific embodiments disclosed in the specification. The scope of the invention is to be determined by the scope of the following claims, and the scope of the following claims will be understood in accordance with the defined principles of the claims.

100. . . Image pixel

110. . . P-type epitaxial layer

115. . . N-type photosensitive region

116. . . Back p+ doped layer

117. . . P-type pinned layer

120. . . Transfer transistor

130. . . N+ source/drain diffusion region

140. . . p well

150. . . Metal stack

154. . . Interlayer dielectric

160. . . Anti-reflection (AR) layer

165. . . Color filter

170. . . Microlens

300. . . Image pixel

305. . . P+ substrate

310. . . Epitaxial layer

315. . . Photosensitive area

316. . . Back p-doped layer

350. . . Metal stack

360. . . Anti-reflection (AR) layer

365. . . Color filter

370. . . Microlens

390. . . Dopant zone

395. . . Cover film

400. . . Back-illuminated (BSI) imaging system

405. . . Pixel array

410. . . Readout circuit

415. . . Functional logic

420. . . Control circuit

500. . . Pixel circuit

C1-Cx. . . Row

FD. . . Floating diffusion node

P1-Pn. . . Pixel

Pa. . . Pixel

Pb. . . Pixel

PD. . . Photodiode

R1-Ry. . . Column

SF. . . Source follower

T1. . . Transfer transistor

T2. . . Reset transistor

T3. . . Source follower transistor

T4. . . Select transistor

VDD. . . Power rail

1 is a cross-sectional view of one embodiment of a back-illuminated image sensor having a backside dopant layer.

2 is a flow chart illustrating an embodiment of a process for fabricating a back-illuminated image sensor.

3A is a cross-sectional view of a partially fabricated embodiment of a back-illuminated image sensor fabricated until the front side of the interconnect is completed.

3B is a cross-sectional view of a partially fabricated embodiment of a backside illuminated image sensor fabricated up to the dopant region and prior to deposition of the polysilicon cap film.

3C is a cross-sectional view of a partially fabricated embodiment of a back-illuminated image sensor fabricated until the back doped layer is completed.

3D is a cross-sectional view of an embodiment of a fabricated back-illuminated pixel.

4 is a block diagram illustrating an embodiment of a back-illuminated image sensor.

5 is a circuit diagram illustrating pixel circuits of two four-electrode ("4T") pixels within an embodiment of a back-illuminated imaging array.

(no component symbol description)

Claims (13)

A method of fabricating a back-illuminated pixel, the method comprising: forming a front side component of the pixel on a front surface of the substrate or in the front surface, the front side component comprising a photosensitive region having a first polarity; Forming a pure dopant region having a second polarity on one of the back faces; applying a laser pulse to the back surface of the substrate to melt the pure dopant region; on the back side of the pure dopant region Depositing a cap film; and recrystallizing the pure dopant region to form a back doped layer, wherein the laser pulse melts the cap film, and wherein the cap film recrystallizes after the laser pulse. The method of claim 1, wherein the cover film is polycrystalline germanium. The method of claim 1, wherein forming the pure dopant region comprises depositing a dopant using a plasma immersion ion implantation ("PIII") process. The method of claim 1, wherein the substrate is an epitaxial substrate having a polarity opposite to the polarity of the photosensitive region. The method of claim 1, wherein the doped layer has the same polarity as the substrate but has a different dopant concentration. The method of claim 1, wherein the laser pulse has a duration of less than about 10 nanoseconds. A pixel comprising: a photosensitive region having a first polarity formed on a front surface of a substrate; a back doped layer formed on a back surface of the substrate, the doped layer being formed from a molten and recrystallized back surface pure dopant region formed on the back surface of the substrate, wherein the back doped layer Further formed by a molten and recrystallized cap film formed on the back surface of the substrate, and wherein the cap film is polycrystalline germanium. The pixel of claim 7, wherein the substrate is an epitaxial substrate having a polarity opposite to the polarity of the photosensitive region. The pixel of claim 7, wherein the doped layer has the same polarity as the substrate but has a different dopant concentration. The pixel of claim 7, wherein the doped layer has a dopant concentration between 10 ¹⁸ ions/cm 3 and 10 ²⁰ ions/cm 3 . The pixel of claim 7, further comprising an anti-reflective coating formed on the back side of the substrate. The pixel of claim 11, further comprising a color filter formed on the anti-reflective coating. The pixel of claim 12, further comprising a microlens formed on the color filter.